Browsing by Subject "Buffet"

This dissertation explores a fluid instability known as buffet, which occurs in the subsonic, transonic, and supersonic regimes. Buffet has been observed in experiments and various computational studies, and its underlying physics are not well-established. The goal of this document is to provide insight into various configurations which produce buffet and attempt to understand the flow physics at play.

Aeroelastic phenomena, and particularly instabilities and Limit Cycle Oscillations (LCOs), remain an important and complex field of research. Understanding of these instabilities is crucial for the design of aerospace vehicles and turbomachinery, where they may impede performance, reduce efficiency, and induce structural fatigue or failure. The subject of this dissertation – transonic buffet – is one such instability. Transonic buffet is a global flowfield instability characterized by an oscillating shock-wave on the suction-side of an airfoil or wing that produces oscillating forces on the structure. The LCO frequencies scale with theairfoil chord, and are near typical natural frequencies for aerospace structures. Theoretical descriptions of buffet’s mechanism remain incomplete, and its onset is difficult to predict without computationally expensive simulations.

This dissertation seeks to explore two-dimensional buffet using Computational Fluid Dynamics (CFD) and provide insight into its sensitivities to both flow conditions and numerical models. Deeper understanding of the two-dimensional behavior is a critical step in exploring the more complex three dimensional phenomenon. Trends in Reynolds number, angle of attack, and Mach number are explored, as are the impact of wind tunnel walls and airfoil geometry. While most studies included in this document make use of Unsteady Reynolds-Averaged Navier-Stokes (URANS), additional methods of simulation are explored and assessed, including inviscid simulations, Implicit Large Eddy Simulation (ILES), and linearized stability analysis. Buffet shows substantial sensitivity to many of the conditions explored in this work. While the buffet onset point, mean lift coefficient, and frequency tend to be relatively stable, the amplitude of oscillation and the conditions near the offset point are especiallysensitive to many factors. Special care must be taken in discretization and selecting models to apply if these highly sensitive outputs are to be explored. Despite these sensitivities, lower fidelity methods, including inviscid simulations and linearized stability analysis, are successful approaches to studying transonic buffet.